The present invention relates to a track-crossing count circuit in an optical disc apparatus, and more particularly, to a track-crossing count circuit for accurately counting the number of track-crossings during data search operations on a disc rotating at high speed.
FIG. 1 shows a conventional track-crossing count circuit for use in an optical disc apparatus.
Referring to FIG. 1, a signal read by a pickup 2 which moves across a disc 1 is input into a radio frequency (RF) amplifier 3 which provides a mirror signal output to an inverter 4. At the same time, an error detector 5 detects the errors of the mirror signal output. A rectangular waveform signal is output from a target speed rectangular waveform generator 6 for controlling the speed of a sled motor 10 as a pickup transferring device. An XOR gate 7 performs an exclusive-OR operation on the mirror signal inverted in a inverter 4, and the rectangular waveform signal, and supplies the result to a track counter 8 which counts the number of track-crossings present in the output signal and provides the counted number to a microcomputer 9. In microcomputer 9, a preset count value is compared with the actual count value in track counter 8. Then, pickup 2 is moved in accordance with the control of sled motor 10 until the set count value coincides with the actual count value. Once the two values coincide with each other, the data on disc 1 is read at the location of pickup 2.
Here, due to the frequency characteristics of the circuit, an AC component decreases in amplitude as the pickup speed increases. Accordingly, when the conventional pickup is moved across the disc at speeds higher than a particular level, the bandwidth of the mirror signal is too broad, so that a track counter 8 cannot accurately count the mirror signal pulses. Even with the mirror signals compensated, the pickup transferring device (i.e., sled motor) is non-linear, such that the target speed rectangular waveform generator still generates errors, which prevents accurate high-speed accessing.